Here’s a better example for the LPC1850 another ARM Cortex M3 MCU I’ve been using lately. In this datasheet they list the input capacitance of the XTAL pins as max 0.8pF (search for Cio p.96 of the 21 Nov 2011 DS):

This is only a simulated value in this case (see note 14) but it should give you a pretty close approximation.

You still need to add in some margin for stray capacitance due to the trace, parasitic capacitance of the capacitors (use smaller physical packages if possible), but honestly just using the rule of thumb above and guessing 2-5pF will already give you far better results that just copying the capacitor values you saw on some other schematic.

Kevin

]]>By: Kevinhttps://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/comment-page-1/#comment-27399
Wed, 25 Jan 2012 15:41:03 +0000http://www.adafruit.com/blog/?p=24343#comment-27399GEorg: It’s true that datasheets aren’t always terribly exhaustive, but you can usually find something close to what you want. I used the LPC1343 a lot (an inexpensive 72MHz ARM Cortex M3 chip from NXP), and they don’t list the input capacitance for the XTAL in/out pins — only ADC inputs are listed at 20pF — but if you look at Table 21 of section 11.2 (XTAL Input) you can at least see a table showing suggested values. Using a 12MHz crystal (which is more or less the de facto crystal for LPC MCUs), they show suggested values for a 10pF or 20pF crystal. If this is all the information you have and accuracy is very important, you’ll be better off picking a 10pF cap instead of the 8pF mentionned above, and then using their suggestion of two 18pF capacitors. You can work backwards of course to guestimate the input capacitance with those values. Or … you can also drop the companies a line if it’s important since someone almost certainly (should!) know this information, though it can be hard to get through the corporate firewall if you don’t know someone inside the company already.
]]>By: GEorghttps://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/comment-page-1/#comment-27394
Wed, 25 Jan 2012 13:33:16 +0000http://www.adafruit.com/blog/?p=24343#comment-27394thanks for this Information – i also found out that quite a number of datasheets don’t give information about the capacity of the XIN Pins ..
]]>By: Kevinhttps://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/comment-page-1/#comment-27381
Tue, 24 Jan 2012 22:16:30 +0000http://www.adafruit.com/blog/?p=24343#comment-27381Georg: That would be included in the stray capacitance … it’s basically a sum of everything between the output of the crystal and the input to the PLL inside the chip, including the parasitic capacitance of any pins or leads on any discrete components, etc. You can look in the datasheets to add all the little bits up, but ~5pF is a pretty good rule of thumb.
]]>By: Georghttps://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/comment-page-1/#comment-27378
Tue, 24 Jan 2012 21:12:43 +0000http://www.adafruit.com/blog/?p=24343#comment-27378I have a further question – don’t you have to take the parasitic input capacity of the pins of the driven chip into account aswell?
]]>By: chuckzhttps://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/comment-page-1/#comment-27365
Tue, 24 Jan 2012 14:31:12 +0000http://www.adafruit.com/blog/?p=24343#comment-27365This is very helpful information. Thank you.
]]>By: mikeselectricstuffhttps://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/comment-page-1/#comment-27362
Tue, 24 Jan 2012 11:44:53 +0000http://www.adafruit.com/blog/?p=24343#comment-27362A handy tip I got from a Microchip guy a while ago is that a ‘happy’ oscillator has a similar amplitude on each side of the crystal. You ideally need to use a low capacitance probe (e.g. x100) to avoid the probe affecting things too much.
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